Transportation and storage of industrial gases such as hydrogen and nitrogen take place with the gas in the form a liquid, the liquefied gas being referred to as a cryogen. Presently, liquid hydrogen is pumped from a storage tank (at 80 to 140 psia) to receivers or tube trailers which store the gas at 2000 to 3000 psia. Such pumping action employs reciprocating pumps which, themselves, cause substantial losses of the pumped cryogen through vaporization. Further, pressure build-ups can result from vaporization of the stored liquid or from vaporization gases from liquid product used to cool the piping and pump prior to pump start up and during operation. Such a pressure build-up can cause a storage tank to exceed its maximum allowable working pressure. In such case, the gas must be vented to the atmosphere and wasted. The benefit which results from the pumping of hydrogen in the liquid state is that the pumping equipment inherently exhibits small displacements and thus equipment size and power requirements are minimized. However, gas vaporization losses offset equipment savings.
U.S. Pat. No. 5,218,827 to Pevzner describes a liquefied gas pumping system wherein a cooling sump is employed. The system design speeds up pump priming, reduces piping heat leakage and eliminates a need for boiling off of the gas to build up tank pressure to compensate for liquid subcooling losses.
U.S. Pat. No. 5,243,821 to Schuck et al. describes a gas delivery system which is designed to provide a supply of gas in a quantity matched to a user's rate of gas consumption. The Schuck et al. system includes a pump/compressor which is adapted to utilize either a vaporized cryogen, a mixture of liquid cryogen and vaporized cryogen, or a subcooled fluid. By varying the gas/liquid composition of the input to the pump/compressor the mass flow rate of the pump/compressor is controlled over a wide range so as to enable a variable gas output feed. Flow control is achieved by the aforementioned varying of the input density of fluids to the pump/compressor through selective feeding of either gas or liquid, or a combination thereof. Control of the mix of gas/liquid fed to the pump/compressor is based upon a comparison of tank discharge pressure versus use pressure; usage demand flow; and temperature at the compression end of the pump/compressor. The Schuck et al. system is specifically designed to provide a variable flow rate of product in accordance with use demands. No attempt is made by Schuck et al. to maximize the output flow rate of gaseous product.
Prior art gas compressors have employed both single and multistage reciprocating compressors to achieve desired levels of gas compression. Input gas flow to the compressors was at ambient temperature, and individual compression stages caused a substantial temperature rise of the compressed gaseous product. As a result, intercoolers and/or aftercoolers were required to assure that gas entering the receivers or tube trailer was substantially at ambient temperature. Such systems required substantial energy inputs to achieve the desired levels of intercooling of the gaseous product.
Accordingly, it is an object of this invention to provide an improved cryogenic gas compression system wherein input gas flow to the compressor is at cryogenic temperature. This results in a maximum output rate for a given first stage displacement.
It is another object of this invention to provide an improved cryogenic gas compression system wherein compressor intercooling facilities are unnecessary.
It is yet another object of this invention to provide a cryogenic gas compression system wherein the compression apparatus operates only upon gaseous product, and boil-off losses are avoided.